The picture below shows a simple FocusRite audio interface with two separate inputs.
- Latency can be an issue when using external audio interfaces. This is when there is a delay between playing the signal and either the computer recording it or hearing it being monitored back.
- This is very off putting when performing and can mean that the ‘feel’ or ‘groove’ of the song is lost and even the performance ending up out of time
- Audio quantise functions can sometimes help to improve this, as can reducing the buffer size on your DAW
- CD quality recordings use a sample rate of 44.1kHz and a bit depth of 16 bits.
- Even some relatively inexpensive audio interfaces can record at higher bitrates, such as 48kHz and 24 bit.
- The Red Book Standard outlines the requirements for digital audio and CDs
- A higher sample rate improves the capture of higher frequencies and thus the higher frequency response. Nyquist’s theorem states that the highest frequency captured is half of the sample rate, and thus to capture the human hearing range, a sample rate of at least 40kHz must be used.
- Benefits of using a higher bit depth include capture of wider dynamic ranges and minimising noise.
- In order to capture or play back digital audio, some conversion needs to take place. Thus, audio interfaces incorporate analogue to digital converters (ADCs) and digital to analogue converters (DACs)
- Audio interfaces will normally incorporate some kind of meter. This shows the volume of the input or output signal and allows the operator to avoid distortion.
|XLR / Jack / Combi Inputs
· There are two combi inputs. These can take both XLR connections and jack connections.
· Jack connections can be TS (tip/sleeve) and TRS (tip/ring/sleeve).
· XLR connections are balanced. This reduces noise using phase cancellation from combining two signals with opposite polarity and the resulting destructive interference
· XLR connections are often used for microphones; jack connections for electric guitars and synthesisers
· There is a locking tab at the top of the connection which means that the XLR cable does not come loose when pulled without pressing the locking tab
· A pre-amp will amplify the input signal as part of the audio interface
|Gain / Level / Pad
· For each input, you can select between instrument and line level.
· The gain control is used to set the input for a good signal to noise ratio and thus reduce noise and prevent distortion
· The pad switch reduces the sensitivity in order to prevent distortion if the signal being recorded is loud. Pad switches often reduce the input signal by up to 20dB.
|Phantom Power / LED Indicators
· The 48V switch activates or deactivates phantom power.
· This is used to supply power to a condenser microphone or active DI box
· The interface only has one switch so this controls phantom power for all channels
· This would create issues if using a condenser and a ribbon microphone as phantom power would break a ribbon microphone
· MIDI stands for Musical Instrument Digital Interface
· It is used as a language to communicate between different electronic instruments
· The LED identifies when a MIDI signal is being sent or received
· The second LED identifies when a USB connection to a computer is active
· Headphones can be used to monitor the signal
· The headphone socket is a TRS/stereo jack connection. It can be used to provide a monitor mix (this may be different to what would be coming out of the monitor speakers).
· On this interface, you can change which output the monitor output mirrors, and use the knob to change the volume of the headphones to ensure a good monitor level and avoid damaging your hearing
· USB stands for Universal Serial Bus
· It allows the interface to be connected to a computer and is used for transferring data
· Power can be provided on a USB cable
· USB 2.0 is a development of the original USB protocol, but is not as fast as ThunderBolt (Apple) and FireWire (IEEE 1394) connections.
· These connectors are 5 pin DIN-180 connectors
· They can be used to connect other equipment such as synthesisers and effects units
· Equipment is connected together in a loop, with the MIDI out of one piece of equipment being connected to the MIDI in of the next piece
· Some interfaces incorporate a MIDI-thru connector, which reduces the latency on the MIDI network
· All sets of outputs here are in stereo, and are analogue
· There are two sets of unbalanced phono / RCA connections.
· The red connector is for the right channel and the white for the left
· Unbalanced connectors are are more susceptible to interference and have a poorer signal-to-noise ratio than balanced connectors
· The two jack outputs are balanced; they are using a TRS jack connector